1. Technical Field
The present invention relates to an optical device, an image projecting apparatus, and an electronic device.
2. Related Art
Recently, there have been commercialized image projecting apparatuses which guide image light emitted by an image display apparatus to the eyes of an observer by using an optical device including a light guide plate, and thereby enable the observer to observe an image (virtual image). For example, for a head-mounted display, a kind of such image projecting apparatus, developments targeted for downsizing, broadening viewing angle, and improving efficiency have been promoted. For such an optical device, heretofore, there has been proposed a method of employing an optical diffraction element as a component for causing light to enter a light guide plate and/or a component for causing light guided along inside the light guide plate to be emitted.
Meanwhile, with respect to a diffraction property of such an optical diffraction element, there is a large wavelength dependency. For this reason, in an image projecting apparatus incorporating an optical device employing such an optical diffraction element, color unevenness of an observed virtual image sometimes increases.
Thus, there has been proposed an optical device including a plurality of optical diffraction elements each associated with a corresponding one of a plurality of color light components (for example, red, green, and blue light components). In an image projecting apparatus employing such an optical device, through a method of causing each of a plurality of color light components which constitute light guided by a light guide plate to be diffracted by a corresponding one of the optical diffraction elements, it is possible to display a virtual image resulting from a reduction of a difference which exists in diffraction efficiency between any two of the color light components and which is caused by the wavelength dependency of the diffraction efficiency (for example, JP-A-2009-186794).
In a configuration disclosed in JP-A-2009-186794, for each of two kinds of components, one for causing light to enter the inside of a wave guide plate, the other one for causing light to be emitted from the inside of the light guide plate, a plurality of optical diffraction elements each associated with a corresponding one of a plurality of color light components which constitute light guided along inside the wave guide plate are layer-stacked. Thus, as the structure of the optical device disclosed in JP-A-2009-186794 has become complex, the manufacturing cost of the image projecting apparatus is likely to increase.
Further, in the case where a plurality of optical diffraction elements each associated with a corresponding one of mutually different color light components are provided in the form of stacked layers, there arises a phenomenon in which light having been diffracted by an optical diffraction element associated with a certain color light component enters an optical diffraction element associated with a different color light component, and consequently the light is subjected to further diffraction. Thus, it becomes difficult to control diffraction angles, and as a result, there have been cases where this difficulty of control has caused a problem, such as degradation of image quality of a displayed virtual image.